Cobalt-doped ZnO nanoparticles and PLD-deposited thin film forms: structure, optical properties and nature of magnetic anisotropy

Cobalt-doped zinc oxide nanoparticles (NPs) were synthesized using a modified sol–gel method. Thereafter, the obtained powder was deposited on a Suprasil glass substrate by employing a pulsed laser deposition (PLD) technique. X-ray diffraction analysis with Rietveld refinement confirmed a hexagonal wurtzite ZnO phase belonging to the P63mc space group for both samples in the NP and thin film forms. In particular, the thin film exhibited an intensive (002) XRD peak, indicating that it had a preferred c-axis orientation owing to the self-texturing mechanism. No segregated secondary phases were detected. The crystallite structure, morphology, and size were investigated using high-resolution transmission electron microscopy (HRTEM). To study the crystalline quality, structural disorder, and defects in the host lattice, we employed Raman spectroscopy. UV-vis-NIR spectroscopy was performed to confirm the nature of the Co-doped ZnO NP powder and the film. The chemical states of oxygen and zinc in the thin film sample were also investigated via X-ray photoelectron spectroscopy (XPS). The M–T curve could be successfully fitted using both the three-dimensional (3D) spin-wave model and Curie–Weiss law, confirming the mixed state existence of weak ferromagnetic (FM) and paramagnetic (PM) phases. Magnetic interaction was quantitatively studied and explained by polaronic percolation of bound magnetic polarons (BMPs). Analysis of magnetic symmetry of the topological antiferromagnetic as-deposited thin film using torque measurements was performed. Based on a phenomenological model, it was revealed that the structure gives rise to uniaxial magneto-crystalline anisotropy (UMA) with the magnetic easy axis parallel to the c-axis.


Introduction
2][3][4][5][6][7][8] This compound possesses a large energy band gap of 3.37 eV and a wide exciton binding energy of 60 meV.It is used in many applications, such as in light-emitting diodes, photodetectors, and gas sensors. 5he theoretical prediction of ferromagnetism at room temperature (RTFM) in ZnO doped with transition metals (TMs) and the discovery of ferromagnetism in pure ZnO and in TM-doped ZnO, such as ZnO doped with Mn, Ni, and Co, have presented an opportunity to utilize these diluted magnetic semiconductors (DMSs) in the eld of magneto-optics and magnetoelectronics (spintronics) technologies.It has been shown that TM-doped ZnO, synthesized using various techniques, displays diverse magnetic characteristics ranging from paramagnetism to ferromagnetism.The Curie temperature (T c ) associated with these materials varies between 20 K and 550 K. 9 Cobalt has attracted signicant interest as a transition metal-doped ZnO.4][15][16] It is worth noting that the cobalt ion Co +2 has a similar radius to that of the Zn +2 ion, resulting in a fortuitous matching of their ionic radii and creating a unique combination of materials.In this scenario, it is anticipated that Co-doped ZnO will possess a crystal structure identical to ZnO and is likely to form a single phase with Co +2 , substituting the Zn +2 positions.6][27] The question of whether the observed RTFM behavior in Zn x Co 1−x O is a result of intrinsic effects (mediated by super-exchange at the atomic scale and charge carriers at a larger scale), or if it is only induced by secondary phases or interfaces remains unanswered.Furthermore, a complete understanding of the relationship between the magnetic and optical behavior in these nanocrystalline particles is also lacking, as contradictory results have been reported in the literature. 28n this contribution, we provide detailed information about the characteristics of 5% Co-doped ZnO, which was synthesized in the form of nanoparticles using a modied sol-gel method, as well as in the form of thin lms deposited on a Suprasil substrate using a pulsed laser deposition (PLD) technique.We aimed to gain a deeper understanding of the factors responsible for the emergence of the ferromagnetic (FM) component in these nanocrystalline particles and to explore the relationship between their magnetic and optical properties.

Experimental
In the initial step, we prepared Zn  O] was added.Aer 15 min magnetic stirring, the solution was placed in an autoclave.Then, it was dried in a supercritical condition of ethyl alcohol (EtOH) (T c = 250 °C, P c = 80 bar) following the protocol described by El Mir et al. 29,30 aer adding 188 mL of EtOH.The produced nanopowder was calcined in an open oven for 2 h at 400 °C.In the second step, Co-doped zinc oxide thin lms were deposited on suprasil substrates by PLD (PLD/MBE 2100) from PVD products obtained using a KrF excimer laser (l = 248 nm, pulse width 20 ns, and repetition rate = 10 Hz) operated at 350 mJ to ablate the target.The deposition temperature and pressure were 300 °C and 10 −6 torr, respectively.The substrate-target distance was 55 mm.All the PLD parameters that could inuence the deposition rate were kept constant during the deposition period. 31The structural properties of the synthesized powder and thin lm were investigated by X-ray diffraction (Bruker AXS B8 Advance using Cu Ka radiation).The chemical composition was investigated by X-ray photoelectron spectrometry (XPS) using a Thermo-Fisher Scientic K-Alpha spectrometer equipped with a monochromatic Al Ka source (hn = 1486.6eV) and a 400 mm X-ray spot size.The morphology of the samples was studied with a JEM-200CX transmission electron microscopy (TEM) system.The Raman spectra were obtained at room temperature using a Microscope confocal Raman Thermo-Fisher DXR system (3 lasers: 532, 633, 785 nm).The optical properties were investigated by UV-visible-IR spectrometry (Shimadzu UV-3101PC) coupled with an integrated sphere in the wavelength range from 200 to 2400 nm.Magnetic measurements were performed by VSM on a Microsense EZ-7 and SQUID Quantum Design MPMS-7X system.The thickness of the lms was evaluated using a Tencor prolometer.The surface morphology and roughness were characterized by atomic force microscopy (AFM, TopoMetrix) and scanning electron microscopy (SEM, HITACHI S4500).

XRD and Rietveld analysis
The Rietveld diagrams for the XRD data of the ZCO NPs powder and the thin lm acquired at room temperature and inputted into the FULLPROF soware 32 are displayed in Fig. 1.It is evident that the as-prepared nanopowder was well crystallized, and could be nely indexed to a wurtzite structure with a pure hexagonal symmetry unit cell (P6 3 mc space group), presenting satisfactory convergence factors.A very precise t among the observed and calculated powder patterns was provided by the Rietveld t.The observed pattern is shown by the dots in Fig. 1(a), while the predicted pattern is shown by the solid line.The lower line shows the discrepancy between the calculated and observed XRD diffractograms.The vertical bars represent the Bragg peak positions.Fig. 1(b) shows the XRD pattern for the thin lm.The diffractogram revealed the existence of a ZCO single phase with a hexagonal wurtzite structure, showing a preferential orientation in the (100) direction with a lowintensity peak, as shown in the inset in Fig. 1(b) and in the (002) direction with the major peak indicating that the c-axis was perpendicular to the lm surface.Furthermore, the preferential orientation along the (002) plane that is evident in the ZCO grown on the suprasil substrate was assisted by the presence of non-bridging oxygen atoms in the substrate. 33The diffraction peak at (002) is a frequently observed phenomenon in ZnO that is obtained with solution-based methods, [34][35][36][37][38] indicating that the (002) plane possessed the lowest surface free energy in the PLD-synthesized ZCO thin lms.Consequently, this plane becomes the preferred orientation for the growth process.The diffraction pattern did not reveal the existence of any phase impurity during the lm growth by the pulsed laser technique.The crystallographic rened structural parameters and reliability factors are provided in Table 1.They agree with the single-crystal X-ray diffraction data and the rened parameters in work previously reported by other groups. 36,37In Fig. 2, a view is provided in perspective of the 3D and polyatomic ZnO nanoparticles to aid a better understanding of the crystal structure with the preferential orientation in the (002) direction, indicating that the c-axis was perpendicular to the lm surface.This explains the spatial arrangement of the (Co/Zn)O 4 octahedra.The crystal structure for the deposited thin lm was established using the "CrystalMaker" program 39 based on the rened atomic positions found by XRD.

Morphological analyses
Fig. 3(a) shows the morphology and an estimation of the particle size according to the TEM image of the ZCO NPs.The particle size was estimated using ImageJ soware via the histogram distribution function.It indicated irregular nanoparticles of various sizes with almost spherical shapes that were uniformly distributed.The majority of the particles revealed an average size of 47 nm.][42] Fig. 3(b) displays the high-resolution (HR) TEM image and an enlarged HRTEM image of the ZCO phase on a structured part of the particle region.The image of the contrast was compatible with the symmetry of the wurtzite ZnO phase nanostructure belonging to the P6 3 mc space group, simulated along the [100] direction.Herein, a schematic reconstruction of the unit cell and Co/ZnO 4 tetrahedral distortions in the (100) axis is shown.The structural representation and the associated models were performed in the crystallographic simulation soware Crystal-Maker. 39 Fig. 2 A view in perspective of the 3D and polyatomic ZnO nanoparticles for better understanding the crystal structure with a preferential orientation in the (002) direction, indicating that the c-axis was perpendicular to the film surface.
presented a uniform grain size distribution with a typical columnar structure, presenting a very smooth surface with a root mean square (RMS) roughness of about 12 nm.The thickness of the lm was about 300 nm, with this value conrmed by Tencor prolometer measurements.

Raman spectroscopic behavior
Raman spectroscopy was employed for the examination and exploration of the growth process of the original crystal structure, the identication of oxygen vacancies, and the detection of localized defects within the nanoparticles.Fig. 5 shows the Raman spectra acquired by exciting the ZCO powder and thin lm nanoparticles by a laser emitting at a wavelength of 532 nm.The examined sample possessed a wurtzite structure and was categorized under the space group P6 3 mc.The phonon mode was weakened on the ZCO powder with respect to the thin lm, but the peak broadening was about the same, suggesting that there was not much deterioration of the lattice.As expected, the thin lm was highly anisotropic, contrary to the powder, which showed a random crystal orientation to the direction of the incoming light.As per group theory, this specic crystallographic phase exhibited optical phonon modes at the center of the zone, which can be denoted as: 43 The aforementioned system was composed of a single A 1 branch, two B 1 branches, one E 1 branch, and two E 2 branches.It is noteworthy that among these branches, E 1 , E 2 , and A 1 could be classied as rst-order Raman active modes, as indicated by previous research. 44Each active vibration mode in the Raman spectra is depicted by a unique band, allowing for the identication and characterization of these modes.The intensity displayed by these bands can be determined by the scattering cross-section of the respective modes. 45The B 1 modes do not demonstrate any infrared or Raman activity.Within this set of modes, both A 1 and E 1 possess polar symmetry and can be divided into transverse optical components known as A 1 (TO) and E 1 (TO), as well as longitudinal optical components called A 1 (LO) and E 1 (LO).The E 2 mode is composed of two distinct modes: E 2 (low) and E 2 (high), which correspond to low-and high-frequency phonons, respectively; E 2 (high) is related with the oxygen atom, while E 2 (low) is linked to the Zn sub lattice.Both E 2 (low) and E 2 (high) modes exhibit Raman activity and are non-polar.Distinct peaks were observed in the ZCO NPs powder at specic wave numbers of 101, 200, 325, 383, 429, 543, 563, 782, and 1121 cm −1 .Similarly, the ZCO thin lm displayed notable peaks at 114, 200, 327, 382, 430, 545, 563, 783, and 1121 cm −1 .Analysis of the results revealed that the ZCO thin lm exhibited a shied peak at 114 cm −1 , suggesting a higher intensity compared to the powder sample.This shi could be attributed to the presence of structural defects, such as local lattice distortion, within the thin lm. 46Additionally, the oxygen defect states were assessed based on the Raman peak intensity ratio, which was found to be approximately 563 cm −1 .The intensity ratios for the ZCO NPs powder and the thin lm were determined to be 0.40 and 0.69, respectively.These results indicate that the presence of oxygen defect states was more prominent in the thin lm form than in the ZCO NPs powder. 47

Optical analyses
The optical properties of the ZCO NPs powder and thin lm form were studied using UV-Vis-NIR absorption (250-2250 nm).As shown in Fig. 6, Co-related absorbance peaks were found between 550 and 700 nm in both the ZCO NPs powder and thin lm sample.The pure ZnO samples displayed an absorption band edge at 393 nm, which suggested a direct bandgap of 3.16 eV. 48The introduction of Co ions in ZnO through the substitution of Zn 2+ by Co 2+ ions yielded three additional absorption bands centered at 568 nm (2.18 eV), 617 nm (2.01 eV), and 660 nm (1.87 eV), respectively.These peaks were related to the d-d (3d 7 ) electronic transition of the tetrahedral-coordinated Co 2+ . 49The three absorption bands could be attributed to 4 A 2 (F) / 2 A 1 (G), 4 A 2 (F) / 4 T 1 (P), and 4 A 2 (F) / 2 E(G) ligand eld transitions, respectively, which involved a crystal-eld split in the 3d levels of Co 2+ substituting for Zn 2+ in ZnO. 50In the NIR region, the NPs powder sample displayed three peaks located at 1315 nm (0.94 eV), 1400 nm (0.88 eV), and 1645 nm (0.75 eV), which were assignable to 4 A 2 (F) / 4 T 1 (F).These absorption peaks in the vis-NIR were attributed to the d-d transition crystal eld from the 4 A 2 (F) state to the higher energy state of Co 2+ , as reported by Koidl et al. 51 Due to the poor signal, these peaks were absent for the thin lm.The optical gap for both samples was evaluated using the Tauc relation (eqn (2)) 52 by plotting (ahn) 2 versus (hn), as shown in Fig. 7 and by the equation below: where A is a constant, a is the absorption coefficient, and hn is the photon energy.The extrapolation of the linear part of the curve to the abscissa axis provides the value of the band gap energy E g of each sample.The expected E g values were 3.04 and 3.34 eV for the NPs powder and thin lm, respectively.4][55] This behavior was not characteristic of the thin lm sample due to the textured state of the ZnO matrix and therefore the absence of defects that are typically responsible for the exchange interactions.

XPS analysis
XPS was used to explore the chemical states of the elements in the lm.The XPS survey scan spectra in Fig. 8(a) showed that all These ndings suggest the presence of Co in the as-grown thin lms in the form of a Co 2+ oxidation state. 57,58Generally, nonstoichiometric phases, including oxygen vacancies, are commonly present and widely observed in oxides, 59 especially in oxide thin lms produced through pulsed laser deposition (PLD).A subsequent annealing in oxygen was required to reduce the oxygen nonstoichiometry.To verify if the intrinsic nature of the exchange interaction in ZCO nanoparticles, which is studied in the following, was induced by oxygen defects, high-resolution XPS measurements were adopted to characterize the situation of oxygen, as displayed in Fig. 8(d).The high-resolution O 1s XPS spectra of the lm could be tted by two peaks: one located

Magnetic characterization
Fig. 9(a) displays the relationship between the magnetization (M) and temperature (T) for a thin lm of ZCO under zero-eldcooled (ZFC) and eld-cooled (FC) conditions.To determine the magnetization value under FC conditions (FCW mode), the sample was cooled from 300 K to 5 K with an applied eld of 1000 Oe.Data were recorded during the subsequent temperature increase.To obtain the magnetization value under ZFC conditions, the sample was cooled from 300 K to 5 K without the presence of any magnetic eld.Again, data were recorded during the temperature increase.Fig. 9(b) shows that the magnetization curves under the FC and ZFC conditions overlapped, but a closer examination reveals that the FC and ZFC magnetization values differed by approximately 120 K and showed a small increase, as depicted in the inset of Fig. 9(b).
The M-T curve demonstrated that ZFC magnetization gradually increased as the temperature decreased from approximately 300 K to around 50 K.However, below 50 K and down to 5 K, the magnetization rapidly increased with further reductions in temperature.Our attempt to t the M-T curve of the ZCO thin lm involved the utilization of the Curie-Weiss (C-W) law, 61 as given by the following equation: where the paramagnetic Curie temperature is symbolized by q and the Curie constant is denoted as C.However, there was a notable disparity between the experimentally observed and theoretically tted curves of the ZCO thin lms (as illustrated in Fig. 9(a)).This discrepancy indicates that the susceptibilities c vs. T did not conform to the Curie-Weiss equation.Consequently, it could be concluded that the ZCO thin lms did not exhibit paramagnetic characteristics within the temperature range of 5 to 300 K. Furthermore, we could not t the M vs. T data of the FC curve of the ZCO thin lms only using a threedimensional (3D) spin-wave model.However, we managed to achieve a successful t of the M vs. T data obtained in the FC state within the temperature range of 5-300 K by employing a combination of the Curie-Weiss and spin-wave models.This was accomplished by applying eqn (4): 62 where A is a coefficient associated to the material structural characteristics, C is the Curie constant and is the paramagnetic Curie temperature, and M 0 is the saturation magnetization at T = 0 K caused by the ferromagnetic component.According to above the applied magnetic eld, 2000 Oe at 10 K, while the primary contributor was diamagnetic (DM) interactions at 300 K. We tted the measured beginning data of M against H curves recorded at 10 K and 300 K in terms of the bound magnetic polaron (BMP) model to understand the applicability of the BMP model for explaining the observed FM contribution. 63ccording to the BMP model, the presence of both correlated (as a result of BMP overlap) and isolated spins reveals how the system got magnetized.The spin-localized charge carriers powerfully interacted with the doped R-ions in the case of a correlated system.Paramagnetism (isolated spin) was caused by the portion of doped Co 2+ ions that were not involved in the BMP interaction (schematically shown in Fig. 11).The measured magnetization may be tted to the relation 64 using this method, as per the following equation.
The rst term in this case represents the contribution from the BMP, while the PM and/or DM matrix is responsible for the second term.Here, M 0 = Nm s , where N is the number-per-unitvolume and m s is the effective spontaneous moment per BMP.Also, m eff is the effective spontaneous magnetic moment of each BMP.As a result, the tting operation was carried out under the assumption that m s = m eff . 65,66In this case, we overlooked the interaction between the BMPs.The number of connected spins rose as a result of the BMP overlap, greatly increasing the magnetism.Previous studies have shown that the TM site in Codoped ZnO does not produce FM. 67The authors conrmed the presence of itinerant electrons in defects, such as oxygen vacancies.Co caused delocalized magnetic moments, which improved the magnetic characteristics.Theoretical investigations showed that Co can contribute signicantly to ferromagnetism by causing a noticeable alteration of the band structure of host oxides. 68,69The exchange interaction resulting among O vacancies and the dopant ions assembles a number of dopant spins around the oxygen vacancies, resulting in the formation of BMPs.As a result, the density of oxygen vacancies is proportional to the number of BMPs. 70The measured rst M vs. H curves and tted ZCO data for the BMP model are shown in Fig. 10(b).The BMP model tted each set of experimental data rather well, and the parameters M 0 , m eff , c m , and N were generated from the model's M vs. H curve tting and are listed in Table 2.The paramagnetic susceptibility (c m ), as determined by the BMP tting, was in the order of 10 −7 in cgs unit, and its value varied slightly with temperature.At 10 K, it was discovered that the effective spontaneous instant per BMP, or m eff , was of the order of 8.914 10 −24 Am 2 , rising to 9.15 10 −21 Am 2 at 300 K.The strength of the polaron-polaron interactions increased with the temperature, which caused a signicant fall in the number of BMPs per unit volume.At 10 K, the total number of BMPs was determined to be in the order of 10 26 cm −3 .However, this concentration was quite high in comparison to the concentration threshold of 10 20 cm −3 required for long-range percolation, 68 which consolidates the idea of the existence of FM interactions among other different components.The presence of Co 2+ in ZnO matrix may lead to an increase in V O .However, the overall numbers of BMPs responsible for longrange FM ordering was still insufficient, suggesting there were several contributors.The V O defect-mediated BMPs and their percolations are important parameters that govern FM behavior.This paper extends the utility of the BMP model and provides an overview of the process governing dilute ferromagnetism.Within our study, we performed a series of angulardependent magnetization scans within the orthogonal plane, perpendicular to the ab plane (Fig. 12(a)).Theses scans were carried out at T = 130 K and 300 K under 500 Oe and 1000 Oe magnetic elds.The angular-dependent magnetic torque  measurements in identical magnetic elds of 500 Oe and1000 Oe at various temperatures are shown in Fig. 12(b).Angulardependent magnetic torque provides information mainly about the magneto-crystalline anisotropy strength.In contrast, the angular-dependent magnetization measurements show the size of the magnetic moment at the given orientation of the sample with respect to the direction of the external magnetic eld.In respect to the hexagonal symmetry, we tted the magnetic torque, using the following formula: 71 where T 0 comes from the background of the torque magnetometer, and T 1,2 are different in magnitude and likely induced the interplay of the (two and four-fold) anisotropy ratio.The coefficients of the two-fold symmetry component sin[2(q M )] are terms for the UMA.The two-fold symmetry of the torque curve indicates a two-fold symmetry of the axis of magnetization (easy and hard).The four-fold symmetry component sin[4(q M )] is a term for a hexagonal system.The estimated values of T 0 for T = 130 K were 1.744(1) 10 −7 Am 2 and 1.185(1) 10 −7 Am 2 for the 500 Oe and 1000 Oe magnetic elds, respectively.For 300 K, they were 1.059(1) 10 −7 Am 2 and 0.795(1) 10 −7 Am 2 for the 500 Oe and 1000 Oe magnetic elds, respectively.Previous reports 72,73 provided a description of the magnetization anisotropy.We carried out analysis of the outof-plane and in-plane hysteresis loops for ZCO thin lms at 130 K and 300 K (Fig. 13).Many theories exist to explain the origin of this anisotropy in magnetization.One possible explanation is the inuence of magneto-crystalline anisotropy on the atomic magnetic moments.In the direction of easy magnetization, the anisotropy works toward maintaining the alignment of the individual spins, resulting in a narrow cone along the easy axis.Consequently, the magnitude of magnetization increases.Conversely, in the hard direction, the anisotropy amplies the angular deviation of spins, causing a wider cone along the hard axis.As a result, the magnitude of magnetization decreases.

Conclusion
The modied sol-gel approach was successfully used to synthesize ZCO nanoparticles.The obtained powder was deposed on a suprasil substrate by a pulsed laser technique (PLD).According to structural investigations using XRD and Raman spectroscopy, the ZCO nanoparticles have a wurtzite structure, with the primitive unit cell being a hexagonal system with the space group P6 3 mc.Additionally, the oxygen vacancies present in ZCO were shown by the peak at 536 cm −1 seen in the Raman spectra.UV-vis-NIR spectroscopy was performed to conrm the nature of the ZnO NPs powder and lm.The chemical states of oxygen and zinc in the ZCO thin lm were also investigated by X-ray photoelectron spectroscopy (XPS).The current work proves that BMPs are produced when oxygen defects result aer annealing in vacuum and Co 2+ -ion doping.
The link between BMPs causes FM in ZCO thin lms.The combination of 3D spin-wave method and Curie-Weiss law provided a good match for the temperature-dependent magnetization curve of ZCO and suggested that paramagnetic and ferromagnetic phases coexist in the range of 5 to 300 K.
Based on a phenomenological model, we revealed that the structure gives rise to uniaxial magneto-crystalline anisotropy (UMA) with the magnetic easy axis parallel to the c-axis.
Fig.4depicts AFM and SEM cross-sectional images of the ZCO thin lm, which demonstrated a preferential crystallographic orientation in the (002) plane, as proved by the highest peak intensity.It could be noticed that the lm

Fig. 1
Fig. 1 Rietveld analysis plots realized with FULLPROF software for X-ray diffraction patterns at room temperature for the (a) ZCO NPs and (b) ZCO thin film.

Fig. 3
Fig. 3 (a) TEM images of ZCO NPs along with their particle size distribution histogram.(b) HRTEM image and an enlarged region showing the structured Co-doped ZnO phase.(c) Schematic reconstruction of the unit cell and Co/ZnO 4 tetrahedral distortions in the (100) axis.

Fig. 5
Fig. 5 Room temperature Raman spectra of both ZCO NPs and the thin film sample.

Fig. 6
Fig. 6 Absorption spectra of both the ZCO NPs and thin film sample.

Fig. 7
Fig. 7 Tauc plots for determining the optical band gap (E g ) for both ZCO NPs and thin film samples.

Fig. 9 (Fig. 9
Fig. 9 Magnetization of the ZCO thin films as a function of temperature in a field of 1000 Oe for (a) zero-field cooling (white circle symbols) and field cooling (black circle symbols).The fitting results of the field-cooled data are shown by the solid curves.(b) Enlarged parts for the temperature range of 25-150 K; the inset shows the relative difference dM/dT vs. T plot.

Fig. 11
Fig. 11 Representation of bound magnetic polarons.Cation sites are shown by small circles.Oxygen is not shown, while the unoccupied oxygen sites are represented by squares.

Fig. 12
Fig. 12 Angular dependence of the magnetic torque measured in 500 Oe and 1000 Oe magnetic fields.The classical plot (a) and polar plot (b)show selected curves at specific magnetic regimes at 130 K and 300 K.

Fig. 13
Fig. 13 Typical magnetic hysteresis loops of ZCO thin films in the range of 0°(out-of-plane) and 90°(in-plane) at (a) 130 K and (b) 300 K.